1. Technical Field
The present invention relates to a method of manufacturing a GRIN lens using a sol-gel process, and a GRIN lens that can be manufactured by the manufacturing method.
2. Background Art
Optical fiber collimators, each including a GRIN lens (Graded Index Lens) fusion-spliced with an end of an optical fiber, can connect semiconductor lasers to optical fibers with high efficiency and can be used as connectors with low coupling loss, or the like, thus being useful as various optical communication parts.
As a method for manufacturing such a GRIN lens, an ion-exchange process, a vapor phase CVD process, or the like is known. A sol-gel process based on low-temperature synthesis is considered to be excellent. For example, Patent Documents 1 to 3 described below each disclose a method for manufacturing a GRIN lens using a sol-gel process. In the method, an acid or base as a solvent is added to an alcohol solution containing a silicon alkoxide (Si(OR)4 (R: alkyl group)) as a main component, hydrolysis is performed to form a sol, and the sol is further subjected to polycondensation, followed by aging, to generate a crosslinking reaction, thereby forming a wet gel. In the production of a GRIN lens, it is necessary to form a concentration distribution in a dopant (i.e., metal component that provides a refractive index distribution). In a portion having a higher concentration of the dopant, the refractive index is higher. Consequently, the GRIN lens is produced such that the central portion has a high concentration of the dopant, and the concentration decreases toward the outer surface. In one method, a metal alkoxide or a metal salt is used as a material for the dopant. Furthermore, a molecular stuffing technique may be used. In particular, use of an alkoxide of Ti, Ta, Sb, or Zr is significantly useful. In order to form a concentration distribution, leaching is generally performed. In the leaching, a wet gel is immersed in an acid solution, and the dopant in the peripheral portion is dissolved away, thus providing a concentration distribution. The resulting wet gel is dried, the solvent in the gel is removed, and then firing is performed to produce a cylindrical, dense glass preform provided with a refractive index distribution. The resulting glass preform is subjected to wire-drawing to reduce its diameter, and thereby, a GRIN lens is produced.
Furthermore, Patent Document 3 described below discloses a technique in which, in the formation of a wet gel, a mixture of a titanium alkoxide and an aluminum alkoxide is added to an alcohol solution containing a silicon alkoxide as a main component to form a wet gel containing aluminum.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-115097
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-145751
Patent Document 3: Japanese Unexamined Patent Application Publication No. 6-122530
In a conventional general sol-gel process, a preform is formed using two components, i.e., silicon and a dopant (for example, when the dopant is Ti, SiO2—TiO2).
In the production of a preform and a lens obtained by wire-drawing the preform, in the case where the dopant concentration is about 10 mole percent, the preform is easily broken during sintering, and the yield in the sintering process is significantly low, which is a problem. Furthermore, in the case where the dopant concentration is 10 mole percent or more, the viscosity at the temperature at which wire-drawing is performed is low, and it becomes difficult to operate, thus decreasing the yield, which is also a problem.
Furthermore, in a GRIN lens having a high numerical aperture with a dopant concentration of 18 mole percent or more, a marked phase separation occurs in the preform during sintering, and it is difficult to obtain a transparent GRIN lens.
One of the means for overcoming such problems is a method in which the composition of the preform is changed from two components to three or more components. In Patent Document 3 described above, aluminum, boron, or germanium is added as a third component. However, many components suitable as the third component are susceptible to acids. Even when an alkoxide is added to a wet gel, most of the alkoxide added is dissolved away by an acid for leaching, and it is difficult to allow an effective amount of the alkoxide to remain in the glass preform.